CMP pad platen with viewport

ABSTRACT

A viewport used by an end point detection system for chemical mechanical polishing (CMP) is attached to a platen that holds a CMP pad. During a CMP operation to polish a semiconductor wafer, the viewport is aligned with a hole in the CMP pad to permit optical access to the wafer by the end point detection system. The viewport shields optical components of the end point detection system from byproducts of the polishing, while also permitting the use of optical techniques to determine how much material has been polished from the surface of the wafer during the CMP operation.

BACKGROUND

[0001] 1. Technical Field

[0002] An embodiment of the invention pertains generally to integrated circuit manufacturing, and in particular pertains to polishing tools used during integrated circuit manufacturing.

[0003] 2. Description of the Related Art

[0004] During fabrication, semiconductor wafers are polished with a process called chemical mechanical polishing (CMP), sometimes referred to as chemical mechanical planarization. CMP includes polishing the wafer with an abrasive object called a pad, using a chemical slurry as a polishing compound. CMP may serve several different purposes, including selective removal or thinning of a surface layer of material from the wafer. Because sub-micron tolerances are involved, the amount of surface material removed (or conversely the amount remaining) must be accurately controlled, or the entire wafer may have to be scrapped. Systems to determine when to stop polishing are called end point detection systems.

[0005] Laser interferometry is sometimes used to make these measurements. To direct the laser light onto the wafer, and to receive reflected laser light from the wafer, some conventional systems place a hole in the pad. However, this hole permits the chemical slurry and particles of removed material from the wafer to contaminate the laser source and laser sensor. One conventional way to prevent this contamination is to use a pad with a transparent portion through which the light can travel. However, this can increase the cost of the pads, a cost which is a repetitive expense since the pads must be frequently replaced due to wear experienced during operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings:

[0007]FIG. 1 shows an exploded view of certain components of a CMP operation, according to one embodiment of the invention.

[0008]FIG. 2 shows a cross section of a platen, pad, and viewport assembled for a CMP operation, according to one embodiment of the invention.

[0009]FIG. 3 shows a flow chart of a CMP operation, according to one embodiment of the invention.

DETAILED DESCRIPTION

[0010] In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known structures and techniques have not been described in detail in order not to obscure the understanding of this description.

[0011] In various embodiments, a viewport is attached to a platen that holds a CMP pad. Within the context of the described embodiments of the invention, a viewport is a device that passes light between optical components of an end point detection system and a wafer being polished in a CMP operation, but which physically shields the optical components from slurry, removed particulate matter, and/or other by-products of the CMP operation.

[0012]FIG. 1 shows an exploded view of certain components of a CMP operation, according to one embodiment of the invention. In the exemplary embodiment of FIG. 1, during a polishing operation, pad 120 may be fastened to platen 110, which is used to hold and rotate pad 120. Because pad 120 must be controlled with precision, but also experiences a great deal of wear, platen 110 may be made and operated with the necessary precision, while pad 120 may be considered a replaceable item that deteriorates with usage.

[0013] In the exemplary embodiment of FIG. 1, pad 120 is a disk-shaped object that rotates about its axis when the underlying platen 110 rotates, as shown. While in the exemplary embodiment of FIG. 2 pad 120 is disk-shaped, alternative embodiments could use other shapes. When the bottom surface of wafer 150 is brought into contact with the top surface of rotating pad 120, the rotation of pad 120 causes the bottom surface of wafer 150 to be polished. (The terms “top” and “bottom” refer to the orientation shown in FIG. 1. The physical orientation of these devices in an actual CMP system may be different.) A chemical slurry may be placed on the top surface of pad 120 to enhance the polishing operation. Since the slower-moving inner portions of the top surface of rotating pad 120 might have less of a polishing effect than the faster-moving outer portions, wafer 150 may be rotated about its own axis during polishing to average out the polishing speed seen by any given point on the bottom surface of wafer 150.

[0014] In the exemplary embodiment of FIG. 1, pad 120 has a hole 140 penetrating from the top surface to the bottom surface, through which an end point detection system may obtain visual access to the bottom surface of wafer 150 during a CMP operation. Platen 110 has a corresponding viewport 130 attached to platen 110. Viewport 130 may be placed over optical components in or below platen 110 that are used in the endpoint detection system. While the illustrated embodiment shows a viewport 130 with a shape having two curved sides and two sides extending radially outward, other embodiments may use a viewport having other shapes. Various embodiments may also have viewports of various sizes.

[0015]FIG. 2 shows a cross section of a platen, pad, and viewport assembled for a CMP operation, according to one embodiment of the invention. FIG. 3 shows a flow chart of a CMP operation, according to one embodiment of the invention.

[0016] In the following text, the operation shown in FIG. 3 and the structures shown in FIGS. 1 and 2 are sometimes described with reference to each other. However, it is understood that the embodiments shown in any of FIGS. 1, 2 and 3 may also be implemented without using the specific embodiments shown in the other figures.

[0017] Referring to FIG. 3, the operation of flow chart 300 begins at block 310 by providing a viewport attached to a platen. In the exemplary embodiment of FIG. 2, viewport 130 is attached to platen 110 with countersunk machine screws 240, with a lip of pad 120 extending over the heads of the screws to prevent any possible contact between screws 240 and wafer 150. In another embodiment, pad 120 has no such lip, but the heads of screws 240 are well below the surface of viewport 130 to prevent contact with wafer 150. Other embodiments may use other forms of attachment (e.g., adhesive or other types of mechanical fasteners) that securely hold viewport 130 to platen 110. While in the illustrated embodiment the points of attachment of the viewport 130 are at the surface of platen 110 (where machine screws 240 enter platen 110), in other embodiments the points of attachment of the viewport 130 may be within a recess in the top of platen 110, e.g., within optical area 230.

[0018] Viewport 130 may be positioned so that viewport 130 covers optical area 230 in platen 110. In the exemplary embodiment of FIG. 2, optical area 230 houses optical components including a light emitter 210 and a light sensor 220. Light emitter 210 and light sensor 220 may be active devices (e.g., a light emitting diode and a light sensing device), or passive devices (e.g., optical fibers that emit and/or receive the light at the ends of the fibers and convey that light through the fibers to/from active components located elsewhere). In the exemplary embodiment of FIG. 2, light emitter 210 and light sensor 220 are positioned some distance below viewport 130, while in another embodiment light emitter 210 and/or light sensor 220 are in intimate contact with viewport 130. While optical area 230 is shown in the exemplary embodiment as an oversized recessed area containing separate optical components, alternative embodiments may use other techniques (e.g., integrating one or more of the optical components into the structure of platen 110, implementing optical area 230 as an opening that passes completely through platen 110 with the optical components extending up through the opening, etc.)

[0019] In one embodiment, light emitter 210 is a laser light emitter that emits a particular wavelength of laser light in the visible light spectrum, while light sensor 220 is a laser light sensor that detects that particular wavelength of laser light. In another embodiment the wavelength of the light is outside the visible light spectrum.

[0020] Whatever the wavelength used, viewport 130 is a device that passes light. While in one embodiment viewport 130 is made of material that is largely transparent to (i.e., passes most or all of) the wavelength of light emitted by light emitter 210, other embodiments may be implemented differently (e.g., with material that is only partially transparent to the light, i.e., the material passes only a portion of the light of the correct wavelength, while still maintaining the other relevant characteristics of the light such as optical coherency).

[0021] Light sensor 220 is positioned such that it receives light from light source 210 that has been reflected back from a wafer 150 positioned immediately above, and in contact with, pad 120. Wafer 150 has been omitted from FIG. 2 to permit the details of the remaining components to be more clearly shown.

[0022] In some embodiments, the interface between viewport 130 and platen 110 forms a seal to prevent slurry, particulates, and other contaminants from migrating from above pad 130 down to the optical components in optical area 230. In one embodiment an O-ring 250 is used to ensure a proper seal, with viewport 130 and/or platen 110 having grooves in their respective surfaces to position the O-ring 250. Other embodiments may provide a seal another way (e.g., non-circular elastomeric rings, mating flat surfaces on viewport 130 and platen 110, etc.)

[0023] With reference to FIG. 3, at block 320 a pad is provided having a hole therethrough, the hole being positioned and shaped such that the viewport fits into the hole when the pad is placed on the platen in an operating position. While in one embodiment viewport 130 is sized to fit tightly within hole 140, in another embodiment viewport 130 is measurably smaller than hole 140, which allows easier insertion but may allow slurry and particulate matter to accumulate between the adjacent sides of viewport 130 and hole 140.

[0024] At block 330 the pad may be coupled to the platen with the viewport aligned with the hole so that optical components in the platen may have visual access to a wafer positioned above the pad. In the embodiment shown in FIG. 2, the bottom surface of pad 120 may be coupled to the top surface of platen 110 in any feasible manner, so that the rotation of platen 110 causes a corresponding rotation of pad 120 as indicated in the exploded view of FIG. 1.

[0025] In the exemplary embodiment of FIG. 2, the top surface of viewport 130 is slightly lower than the top surface of pad 120. Since the pad material may be compressible, this difference in height permits the pad 120 to be compressed during a CMP operation without having the viewport 130 come in contact with the wafer 150. Such contact could potentially damage the wafer. Such contact could also mar the surface of the viewport 130 and degrade its optical qualities to the detriment of the end point detection system. In another embodiment, viewport 130 may be made out of material with similar compressibility and/or surface characteristics as the material in pad 120, so that contact between viewport 130 and wafer 150 will cause no harm. In this embodiment, the surface of viewport 130 may be even with the surface of pad 120. In still another embodiment, the compressibility of viewport 130 is at least partially provided by compressibility of viewport 130's supporting structure, such as an oversized O-ring 250 that keeps viewport 130 raised above platen 110 except when pressure on viewport 130 compresses the O-ring.

[0026] Returning to FIG. 3, but with reference to the structure of FIGS. I and 2, at block 340 a CMP operation is begun by polishing the wafer 150 with the pad 120 by rotating pad 120 and wafer 150 about their respective axes. As can be seen in FIG. 1, during each rotation of pad 120, hole 140 and viewport 130 will pass under wafer 150, during which time a measurement may be made by directing light from the light emitter 210 onto the pad and measuring the light reflected from the pad with the light sensor 220. In one embodiment measurement is an iterative process, with successive measurements from successive passes being used to determine the changing thickness of the surface material on the wafer being polished, or alternatively determining the amount of material removed since the previous measurement. The amount of surface area on wafer 150 that is exposed to hole 140 during each pass depends on the rotational speed of wafer 150 compared to the rotational speed of pad 120. In a particular embodiment, the center of hole 140 passes under the center of wafer 150, and the relative rotational speeds result in every portion of the bottom surface of wafer 150 being exposed to hole 140 a minimum number of times during a CMP operation. This permits every portion of wafer 150 to be measured by the end point detection system.

[0027] To ensure that the polishing slurry and particulate matter does not prevent reliable operation of the end point detection system by covering up viewport 130, the surface of viewport 130 may be periodically wiped with a squeegee or other suitable device during the CMP operation. While in one embodiment the top surface of both the viewport 130 and the pad 120 are wiped off once with each rotation of pad 120, in other embodiments other wiping techniques may be used.

[0028] As indicated in FIG. 3, and with reference to the structure of FIGS. 1 and 2, at block 350 light from light emitter 210 is directed through the viewport 130 and the hole 140 to the bottom surface of wafer 150, and at block 360 that light is detected by light sensor 220 after being reflected from wafer 150 back through the hole and the viewport. In one embodiment, the light is laser light of a particular wavelength, and differential phase techniques are applied to the received light signal to determine the thickness of a layer of material on the surface of wafer 150. In other embodiments, other techniques may be used measure the thickness of the surface layer of material.

[0029] Based on the detected light signal, at block 370 it is determined whether enough material has been polished off to produce a predetermined thickness of the surface material on the wafer. If not, blocks 350-360 are repeated. If yes, the CMP polishing operation may be stopped. In one operation, the thickness is determined by taking measurements at various random locations on the wafer and averaging them together until the average reaches a predetermined number. In another operation, the thickness is determined by measuring specific locations on the wafer and continuing until every location falls within a specified range. Other operations may use other techniques.

[0030] Embodiments of the invention may be implemented in one or a combination of hardware, firmware, and software. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by at least one processor to perform the operations described herein. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computer). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic disk storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others.

[0031] In a particular embodiment, the instructions referred to above may direct a CMP polishing system to polish a wafer by taking optical measurements through a viewport attached to a platen in the manner described for blocks 340-380 of FIG. 3.

[0032] The foregoing description is intended to be illustrative and not limiting. Variations will occur to those of skill in the art. Those variations are intended to be included in various embodiments of the invention, which are limited only by the spirit and scope of the appended claims. 

I claim:
 1. An apparatus, comprising: a platen to hold a pad to be used in a chemical mechanical polishing operation on a wafer; and a viewport attached to the platen to permit optical measurement of the wafer through the pad during the chemical mechanical polishing operation.
 2. The apparatus of claim 1, wherein: the viewport is to fit within an opening through the pad.
 3. The apparatus of claim 1, wherein: the viewport is removably attached to the platen.
 4. The apparatus of claim 1, wherein: the viewport is attached to the platen with a threaded fastener.
 5. The apparatus of claim 1, wherein: the viewport is attached to the platen with an adhesive.
 6. The apparatus of claim 1, further comprising: an O-ring seal between the platen and the viewport.
 7. The apparatus of claim 1, wherein: the viewport has a compressibility substantially equivalent to a compressibility of the pad.
 8. The apparatus of claim 1, wherein: the viewport includes a material that is at least partially transparent at a wavelength of light used in the optical measurement.
 9. A system, comprising: a pad to polish a wafer in a chemical mechanical polishing operation, the pad having an opening therethrough; a platen coupled to the pad; a viewport attached to the platen and disposed at least partially within the opening; and a light emitter coupled to the platen to direct light to the wafer through the viewport and through the opening during the chemical mechanical polishing operation.
 10. The system of claim 9, wherein: the light emitter is a laser light emitter.
 11. The system of claim 9, wherein: the viewport is at least partially transparent to the light.
 12. The system of claim 9, wherein: the viewport is removably attached to the platen.
 13. The system of claim 9, wherein: the viewport is attached to the platen with a threaded fastener.
 14. A method, comprising: polishing a wafer with a pad coupled to a platen during a chemical mechanical polishing operation; and directing light to the wafer through a viewport attached to the platen to determine a thickness of a surface material on the wafer.
 15. The method of claim 14, wherein: said directing includes directing the light through an opening in the pad.
 16. The method of claim 14, wherein: said directing includes directing the light through an opening in the pad in which the viewport is disposed.
 17. The method of claim 14, wherein: said polishing includes at least one of removing a first predetermined thickness of surface material and removing the surface material until a second predetermined thickness of surface material remains.
 18. A machine-readable medium that provides instructions, which when executed by a set of one or more processors, cause said set of processors to perform operations comprising: directing light to a wafer through a viewport attached to a platen to determine a thickness of a surface material on the wafer as the surface material is being polished by a pad in a chemical mechanical polishing operation; and determining when to end the chemical mechanical polishing operation based on the thickness of the surface material.
 19. The medium of claim 18, wherein: said directing includes directing the light through an opening in the pad in which the viewport is disposed.
 20. The medium of claim 18, wherein: said directing includes directing the light through the viewport attached to the platen with a mechanical fastener. 